U.S. patent application number 10/432579 was filed with the patent office on 2004-05-27 for fluid mixing system.
Invention is credited to Rondeau, Joel, Vigneaux, Pierre.
Application Number | 20040100858 10/432579 |
Document ID | / |
Family ID | 24920002 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040100858 |
Kind Code |
A1 |
Rondeau, Joel ; et
al. |
May 27, 2004 |
Fluid mixing system
Abstract
A method for continuously mixing a borehole fluid such as cement
includes using a measurement of the solid fraction of a cement
slurry as it is being mixed to determine the ratio of the solid and
liquid components to be added to the slurry. A system for mixing
the includes a liquid material (water) supply including a flow
meter; a solid material (cement) supply; a mixer which receives the
liquid and solid materials and includes an output for delivering
materials from the mixer to a delivery system; a device for
measuring the amount of material in the mixer, and a flow meter in
the output; wherein measurements from the flow meters and the
device for measuring the amount of material in the mixer are used
to control the amount of solid and/or liquid material added to the
mixer.
Inventors: |
Rondeau, Joel; (Antony,
FR) ; Vigneaux, Pierre; (Moisenay, FR) |
Correspondence
Address: |
SCHLUMBERGER TECHNOLOGY CORPORATION
IP DEPT., WELL STIMULATION
110 SCHLUMBERGER DRIVE, MD1
SUGAR LAND
TX
77478
US
|
Family ID: |
24920002 |
Appl. No.: |
10/432579 |
Filed: |
November 18, 2003 |
PCT Filed: |
October 4, 2001 |
PCT NO: |
PCT/EP01/11483 |
Current U.S.
Class: |
366/19 ; 366/136;
366/141; 366/152.2; 366/152.6; 366/153.1 |
Current CPC
Class: |
B01F 35/20 20220101;
B01F 25/53 20220101; B01F 35/2134 20220101; B28C 7/022 20130101;
B01F 35/2132 20220101; B01F 35/213 20220101; B01F 2101/49 20220101;
B01F 35/2117 20220101; E21B 21/062 20130101; G05D 11/132 20130101;
B01F 23/59 20220101; B01F 35/2111 20220101; B01F 35/2112 20220101;
B01F 2215/0409 20130101; B01F 35/2211 20220101; B01F 2101/28
20220101; G05D 11/135 20130101; E21B 33/13 20130101; B01F 35/881
20220101; B01F 35/21112 20220101 |
Class at
Publication: |
366/019 ;
366/136; 366/141; 366/152.2; 366/152.6; 366/153.1 |
International
Class: |
B01F 015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2000 |
US |
09/726784 |
Claims
1 A system for mixing a cement slurry comprising: i) a liquid
material supply (101) including a device (104) for measuring the
amount of liquid material supplied; ii) a solid cement supply
(108); iii) a mixer including a tub (114) which receives the liquid
and solid cement and includes an output (124) for delivering
materials from the mixer to a delivery system; iv) a flow meter
(128) in the output (124); and v) a computing device, characterised
in that the system further comprises a device (120) for measuring
the amount of material in the mixer; the computing device receives
the output from the device (120) for measuring the amount of
material in the mixer and from the flow meters (104, 128) and is
arranged to calculate the variation in the amount of material in
the tub (114) over time, and the solid fraction of the mixture; and
the calculated solid fraction is used to control the relative
amounts of solid and liquid material added to the mixer.
2 A system as claimed in claim 1, wherein the flow meters are
selected from mass flow meters and volumetric flow meters.
3 A system as claimed in any preceding claim, wherein the flow
meters are selected from Coriolis meters and electromagnetic
meters.
4 A system as claimed in any preceding claim, wherein the mixer
comprises a mixing section and the tub, mixed materials being fed
from the mixing section to the mixing tub, and a portion of the
materials in the tub being recirculated to the mixing section.
5 A system as claimed in claim 4, wherein the device comprises a
level sensor in the tub.
6 A system as claimed in claim 4, wherein the device comprises a
load sensor which measures the weight of the tub.
7 A system as claimed in any of claims 4-8, wherein recirculation
of material takes place upstream of the flow meter in the
output.
8 A system as claimed in any preceding claim, comprising separate
supplies of cement and dry additives, a flow meter being provided
to measure the rate of flow of the dry additives.
9 A system as claimed in claim 8, wherein the supply of dry
additives comprises multiple separate supplies of additives, each
with its own flow meter.
10 A system as claimed in any preceding claim, wherein the liquid
supply includes at least one tank.
11 A system as claimed in claim 10, wherein the device for
measuring the amount of liquid supplied comprises a level sensor in
the tank or a flow meter which measures the amount of liquid
flowing from the tank.
12 A method of mixing a cement slurry, wherein the cement and
liquid components are continuously delivered to a mixer and the
cement slurry is continuously removed from the mixer for use, the
method comprising: i) measuring the flow rate of liquid components
into the mixer; and ii) measuring the flow rate of slurry removed
from the mixer; characterised by iii) measuring the amount of
slurry in the mixer; iv) using the measurements of flow rate and
amount of slurry to calculate the solid fraction of the fluid; and
(v) controlling the delivery of cement and/or liquid components to
the mixer according to the calculated solid fraction.
13 A method as claimed in claim 12, wherein additives are delivered
to the mixer separately from cement, the method further comprising
measuring the flow rate of the additives delivered to the
mixer.
14 A method as claimed in claim 12 or 13 wherein the mixer includes
a tub, the measurement of the amount of slurry in the mixer
comprising a measurement of the amount of slurry in the tub.
15 A method as claimed in claim 14, wherein a portion of the slurry
in the tub is recirculated into the mixer as solid and liquid
components are added.
16 A method as claimed in claim 15, wherein the recirculation takes
place upstream of the measurement of the flow rate of slurry
removed from the mixer.
17 A method as claimed in any of claims 12-16, wherein liquid
components are delivered for mixing from a liquid supply including
at least one tank, the method further including the steps of
measuring the level of liquid in the tank over time and computing
displacement volume as: .SIGMA.(V(h1n)-V(h2n)) where: V(h) is the
exact volume of the tank at level (h) h1n is the start level of the
n th displaced tank volume; and h2n is the stop level of the n th
displaced tank volume.
Description
[0001] The present invention relates to a system for mixing fluids
containing solid and liquid materials such as cement. In particular
the invention provides a system for the continuous mixing of
cements or other fluids used in the drilling, completion or
stimulation of boreholes such as oil or gas wells.
[0002] When a well such as an oil or gas well has been drilled, it
is often desired to isolate the various producing zones form each
other or from the well itself in order to stabilise the well or
prevent fluid communication between the zones or shut off unwanted
fluid production such as water. This isolation is typically
achieved by installing a tubular casing in the well and filling the
annulus between the outside of the casing and the wall of the well
(the formation) with cement. The cement is usually placed in the
annulus by pumping a slurry of the cement down the casing such that
it exits at the bottom of the well and passes back up the outside
of the casing to fill the annulus. While it is possible to mix the
cement as a batch prior to pumping into the well, it has become
desirable to effect continuous mixing of the cement slurry at the
surface just prior to pumping into the well. This has been found to
provide better control of cement properties and more efficient use
of materials.
[0003] The cement slurries used in such operations comprise a
mixture of dry and liquid materials. The liquid phase is typically
water and so is readily available and cheap. The solid materials
define the slurry and cement properties when added to the water and
mixed, the amount of solid materials in the slurry being important.
Since the liquid phase is constant, the amount of solid material
added is usually monitored by measuring the density of the slurry
and maintaining this at the desired level by controlling the amount
of the solid material being added. FIG. 1 shows a schematic diagram
of a prior art mixing system. In the system of FIG. 1, mix water is
pumped from a feed supply 10 via a pump 12 to a mixer 14 which
feeds into a mixing tub 16. The feed supply 10 comprises a pair of
displacement tanks 11, 11' each with separate outlets connected to
a valve 13 which in turn feeds the pump 12. Two methods are
commonly used to determine the amount of water supplied:
[0004] 1. Proximity switches installed on the shaft of the pump 12
count a number of pulses per rotation. Each pulse corresponds to a
displacement volume. This method is sensitive to pump
efficiency.
[0005] 2. Displacement volume is measured by counting the number of
tanks pumped down-hole. This measurement method is sensitive to
human error in level reading, switching from on tank to another and
tank exact capacity. Even more an error in the number of tanks
counted can have many consequences (over displacement can result in
wet shoe, under displacement can result in no pressure bump or
cement left in the casing).
[0006] Solid materials are delivered to the mixer 14 from a surge
can 18 or directly from a cement silo via a flow control valve 20
and are carried into the mixing tub 16 with the mix water. The
contents of the mixing tub 16 are recirculated through a
recirculation pipe 22 and pump 24 to the mixer 14. The
recirculation pipe 22 also includes a densitometer 26 which
provides a measurement of the density of the slurry in the mixing
tub 16. An output 28 is provided for slurry to be fed from the
mixing tub 16 to further pumps (not shown) for pumping into the
well. Control of the slurry mixture is achieved by controlling the
density in the mixing tub 16 as provided by the densitometer 26 by
addition of solid material so stay at a predetermined level for the
slurry desired to be pumped. The densitometer 26 is typically a
non-radioactive device such as a Coriolis meter.
[0007] While this system is effective for slurries using materials
of much higher density than water, it is not effective for slurries
using low density solid materials, especially when the density of
the solids is close to that of water. In such cases, a density
measurement is not sensitive enough to control the amounts of solid
material added to the necessary accuracy.
[0008] The present invention seeks to provide a mixing system which
avoid the problem of density measurement described above.
[0009] In its broadest aspect, the present invention comprises
using a measurement of the solid fraction of a fluid as it is being
mixed to determine the ratio of the solid and liquid components
added to the slurry.
[0010] The invention is particularly applicable to the mixing of
borehole cement slurries, in which case, solids fraction is.
determined as (slurry vol-water vol)/slurry vol. An alternative but
related parameter is porosity, determined as water vol/slurry vol
(porosity+solids fraction=1).
[0011] A system for mixing cement in accordance with the invention
comprises a liquid (water) supply including a device for measuring
the amount of liquid supplied; a solid material supply; a mixer
which receives the liquid and solid materials and includes an
output for delivering materials from the mixer to a delivery
system; a device for measuring the amount of material in the mixer;
and a flow meter in the output; wherein measurements from the flow
meters and the device for measuring the amount of material in the
mixer are used to control the amount of solid material added to the
mixer.
[0012] The flow meters can be mass flow meters or volumetric flow
meters. Any suitable form of meter can be used, for example
Coriolis meters or electromagnetic meters.
[0013] The mixer will typically include a tank or tub, in which
case the device for measuring the amount of material in the mixer
can be a level sensor. Such a level sensor is preferably a time
domain reflectometry- or radar-type device although acoustic or
float devices can also be used. It is preferred to mount such a
device in an arrangement for damping transient fluctuations in the
tank level, for example in an arrangement of concentric slotted
tubes. An alternative or additional form of sensor can be a load
cell which can be used to indicate the weight of the tank, or a
pressure sensor.
[0014] The device for measuring the amount of liquid supplied can
be a flow meter or a level sensor of the types described above.
When the liquid supply includes one or more displacement tanks, a
level sensor is preferred
[0015] Where the mixer includes some form of recirculation of the
slurry through the tank, it is important that the output flow meter
is downstream of this recirculation.
[0016] Where the solid materials comprise cement and other solid
additives added separately to the mixer, separate flow meters can
also be provided for each separate supply of additives.
[0017] In its simplest form, the measurement of solid fraction is
used as a guide for the operator to add solids, particularly
cement, to the slurry as it is mixed. In more advanced versions,
the calculation of solids fraction is used to control the addition
of solids directly by means of an automatic control system.
[0018] The invention also provides an improved method for
calculating displacement volume from a system comprising at least
one displacement tank, comprising measuring the level of liquid in
the tanks over time and calculating displacement as:
.SIGMA.(V(h1n)-V(h2n))
[0019] where:
[0020] V(h) is the exact volume of the tank at level (h);
[0021] h1n is the start level of the n th displaced tank volume;
and
[0022] h2n is the stop level of the n th displaced tank volume.
[0023] Examples of the present invention will now be described with
reference to the accompanying drawing, in which:
[0024] FIG. 1 shows a prior art mixing system;
[0025] FIG. 2 shows a mixing system according to a first embodiment
of the invention;
[0026] FIG. 3 shows the components of a tank level sensor;
[0027] FIG. 4 shows the components of the level sensor
assembled;
[0028] FIG. 5 shows a schematic of the tank level measurement;
and
[0029] FIG. 6 shows a mixing system according to a second
embodiment of the invention.
[0030] The system shown in FIG. 2 is used for the continuous mixing
of cement for oil well cementing operations and comprises a supply
of mix water 100 feeding, via a pump 102 and a flow meter 104 to a
mixing system 106.
[0031] The supply of mix water comprises a pair of displacement
tanks 101, each having a separate output connected to a valve 103
which supplies the pump 102. Level sensors 105 are included in each
displacement tank 101 for determining the amount of water supplied
to the pump 102. In another version (not shown), the level sensors
are omitted. The amount of water supplied is determined in the
manner described below.
[0032] The mixing system 106 also receives solid materials from a
surge can 108 (or alternatively directly from a surge can) which
are admitted through a valve 110. The mixed solid and liquid
materials are delivered through a feed pipe 112 to a mixing tub
114. The mixing tub 114 has a first outlet 116 connected to a
recirculation pump 118 which feeds the slurry drawn from the tub
114 back into the mixing system. The tub 114 is provided with a
level sensor 120 and/or a load sensor 122 to provide an indication
of the tank contents and any change in contents over time. A second
output 124 is provided from the tub 114 which leads, via a second
pump 126 and a second flow meter 128 to the pumping system from
which it is delivered to the well (not shown). An alternative
method of delivery (shown in dashed line in FIG. 2) has an output
124' taken from the recirculation line via a flow meter 128' to the
well. Other arrangements are also possible. The pumps 102, 118, 126
are of the usual type found in well cementing systems, for example
centrifugal pumps. Likewise, the flow meters 104, 128' are
conventional, for example Coriolis meters such as those that have
been used as densitometers in previous applications. Different
types of pumps and meters each have advantages and disadvantages
that are well known in the art and can be selected according to
requirements.
[0033] FIGS. 3-5 show details of the level sensors used in the
displacement tanks and tub and the manner of installation. The
sensor comprises a Krohne radar sensor 200, a stainless steel rod
202, an inner slotted sleeve 204 and an outer slotted sleeve 206.
The rod 202 is screwed onto the sensor 200 and the inner sleeve 204
mounted over the rod 202 and attached to a flange on the sensor
200. The outer sleeve 206 is mounted over the inner sleeve 204 to
which it is attached.
[0034] For use in the displacement tanks, each displacement tank
receives a level sensor. This sensor gives an accurate measurement
of the liquid level in the tank. The exact volume versus level is
required to calculate the displaced volume. In case the tank cross
section profile is not accurately known a so-called tank
calibration is performed. A water meter equipped with a digital
output measures the exact displacement tank volume versus tank
level. This operation is performed only once for each tank. To
supply water to the system, the valve 103 is operated to allow
water to flow from one or other tank to the pump 102. When a tank
discharge valve is opened, a device such as an end switch, pressure
switched or any other appropriate device is used to begin
calculation of the displacement volume. Displacement volume is then
computed as:
.SIGMA.(V(h1n)-V(h2n))
[0035] Where:
[0036] V(h) is the exact volume of the tank at level (h)
[0037] h1n start level of the n th displaced tank volume
[0038] h2n stop level of the n th displaced tank volume
[0039] When the level in the tank in use becomes low, the supply is
switched to the other tank. Switching operation from one tank to
another can either be manual or automated and when one tank is
emptying the other one is filled up for further use. Since the
level sensors can be used to give an instantaneous measurement of
the amount of water provided to the system, it is possible to
confirm the data provided by the flow meter 104, or even to replace
the need for this flow meter completely. When the flow meter is
present, it is not essential to have the level sensors in the
displacement tanks.
[0040] This method of determining the displacement volume can be
applied to other forms of cementing operation than the ones
described here, and has the advantage that it is relatively
insensitive to pump efficiency or operator error as found in the
previous systems.
[0041] For use in the mixing tub, the sensor arrangement is
installed in the mixing tub 114 in the vertical position and in a
location where the slurry is renewed as the mixing occurs, to avoid
location in a dead zone where cement might set. The sensor provides
a measurement of the difference between the length of the rod 202
(LM) and the level of slurry in the tub level (TL). The free tub
level (FTL) is obtained by:
FTL=LM-TL.
[0042] It will be appreciated that the exact form of level sensor
is not important to the overall effect of the invention. What is
important is to obtain an indication of the variation versus time
of the tub slurry volume (called "tub flow" in this document). This
can be obtained using a float or a load sensor or combinations of
any of these or any other sensor giving this information.
[0043] The outputs of the flow sensors and level sensors are used
to monitor the solid fraction of the slurry in the following
manner:
[0044] The solid fraction computation is based on the balance
between incoming and outgoing volumes (or flow rates) as expressed
in the following relationship:
Q.sub.water+Q.sub.cement=Q.sub.slurry+Q.sub.tub
[0045] where Q.sub.tub is the tub rate.
[0046] Tub rate is the variation versus time of the tub volume and
is considered as positive while the tub level increases and
negative while it decreases. The smaller the tub cross section, the
more sensitive the measurement will be to change. Q.sub.tub is
given by: 1 Q tub = S tub h tub t
[0047] where S.sub.tub is the tub cross section and 2 h tub t
[0048] is the tub level variation over time. In the simplest case,
the tub section is constant and the tub rate becomes the product of
the tub level variation/time and the tub cross section.
[0049] The solids fraction at time t is computed as the ratio of
(slurry vol-water vol) over the total slurry volume present at time
t in the tub. The variation in tub slurry volume
V.sub.tub(t+.delta.t)-V.sub.tub(t) can be expressed as:
V.sub.tub(t+.delta.t)-V.sub.tub(t)=.left
brkt-bot.Q.sub.water(t)+Q.sub.cem- ent(t)-Q.sub.slurry(t).right
brkt-bot.*.delta.t
[0050] which can be rewritten as:
V.sub.tub(t+.delta.t)-V.sub.tub(t)=Q.sub.tub(t)*.delta.t.
[0051] In the same way, the variation in the water volume present
in the tub at time t V.sub.water(t+.delta.t)-V.sub.water(t) is
equal to the incoming water volume minus the amount of water
present in the slurry leaving the tub, and can be expressed as:
V.sub.water(t+.delta.t)-V.sub.water(t)=.left
brkt-bot.Q.sub.water(t)-(1-So-
lidFraction(t))*Q.sub.slurry(t).right brkt-bot.*.delta.t.
[0052] Solid Fraction is then expressed as: 3 SolidFraction ( t + t
) = 1 - V water ( t ) + Q water ( t ) - ( 1 - SolidFraction ( t ) )
* Q slurry ( t ) * t V tub ( t ) + Q tub ( t ) * t
[0053] The calculation requires that the initial conditions be
known if it is to be accurate tab initio, i.e. is the tub empty,
full of water or containing slurry already. The calculation will
ultimately stabilise independently of the initial conditions, the
time taken to do this depending on the tub volume and the output
flow rate Q.sub.slurry.
[0054] These calculations are conveniently performed using a
computer, in which case the measurements can be provided directly
from the sensors via a suitable interface. A preferred screen
display will show the various flow rates or levels, together with
the desired solids fraction (calculated when designing the slurry).
The mixing process is controlled by adjusting the amount of cement
and/or water added to the mixer so as to maintain the calculated
solids fraction at the desired level. Alternatively, the results of
the calculations can be fed to an automatic control system which
adjusts the rate at which the components are delivered to the
mixing system.
[0055] The system described above works well when the dry
ingredients (blend of cement+additives) are delivered pre-mixed to
the well site from another location. In this case essentially the
same measurements and calculations as described above are
performed, merely substituting Q.sub.blend for Q.sub.cement. If it
is desired to mix the dry materials on site as part of the
continuous mixing process, a slightly different approach is
required. FIG. 6 shows a mixing system according to another
embodiment of the invention and uses a numbering scheme which
follows that of FIG. 2. The system of FIG. 6 comprises an
additional dry material supply 130 which admits the dry products to
the mixing system 106 via a mass flow meter 132 (other flow
measurement means can also be used) and a control valve 134. In
this case, the basic control equation becomes:
Q.sub.water+Q.sub.additive+Q.sub.cement=Q.sub.tub+Q.sub.slurry
[0056] where four of the five variables are know and Q.sub.cement
is the most difficult parameter to measure accurately. Where
multiple dry additives are to be added, the supply can comprise
separate material supplies, each with a flow meter and valve.
Additional terms Q.sub.additive1, Q.sub.additive2, etc., are
included in the control equation.
[0057] It will be appreciated that changes can be made in
implementation while still remaining within the scope of using
solid fraction as the property monitored to effect control of the
mixing.
[0058] For example, the method can be applied to the mixing of
other borehole fluids such as stimulation fluids (fracturing fluids
or even drilling fluids (mud). In the case of fracturing fluids,
the gel and proppant (liquid and solid phases) are usually mixed
using a pod blender and the proportion of gel and proppant
controlled using a densitometer (usually radioactive) downstream of
the mixer/blender. The use of radioactive sensors generates many
environmental issues and while Coriolis-type meters are an
alternative, they are know to have limitations in respect of flow
rate when used this way. The present invention allows control of
proppant and gel concentrations by means of flow meters without the
need to rely on densitometer measurements.
[0059] Gel and mixed fluid flow rates are measured by means of
electromagnetic flow meters. The amount of proppant is directly
deduced from the following relationship:
Q.sub.gel+Q.sub.Proppant=Q.sub.MixedFluid
[0060] Proppant concentration (in Pounds Per Gallon Added or "PPA")
can be a function of solid fraction as defined above and expressed
as the following:
PPA=Proppant Density*Solid Fraction/(1-Solid Fraction).
[0061] Thus the solid fraction measurement methodology described
above in relation to cement can be applied to fracturing fluids by
determining proppant density rather than cement density.
[0062] This approach has the advantage of not requiring the use of
radioactive densitometers thus avoiding limitations placed on use
for regulatory reasons and without the flow rate performance
limitations of other measurement techniques. The equipment and
control system is essentially the same as that used in the
cementing system described above.
* * * * *